High density beam tube



K. SFANGENBERG ETAL 12,436,833

March -2, 1948;

HIGH DENSITY BEAM TUBE Filed June 15, 1942 3 SheetS-Shet 1 l6 01s TANGEo/v AX/s 9p 7 o s 4 P/u'ssuRs mm mv mo KARL JPENGJVBERG ROBERT new ByLESTER M. FIELD ATTORNEY 55AM SPREAD R/ March 2, 1948.

K. SPANGENBERG ETAL HIGH DENSITY BE TUBE Filed June 15,1942

3 Sheets-Sheet 3 5 6/001. mmsu/ zza 6000;. 6422014750- I 370m. Mmsunza,|6sa76 s+ a 2 [#957554 a 2. K'7887654 PRESSURE r 4 mvem'ok KARLSPANGfNBERG wear/4am BY LESTER Milan ATTORNEY.

Patented Mar. 2, 1948 UNITED STATES PATENT. OFFlC HIGH BEAM TUBE KarlSpangenberg, Lester M. Field; and Robert Helm, Palo Alto, Calif.,assignors to International Standard Electric Corporation, New York, N.Y.. a corporatlonof Delaware Application- June 15 1942', Serial No.447,194

.This invention relates to electron beam tubes, more particularly'totubes such as the velocity modulation type wherein the electron beamtraverses a field-free space for a comparatively great distance;

In tubes of these types it is'desirable to produce an electron beam ofhigh density, that is a beam' haVing as little spread as possible,somewhat analogously to the production of a high concentrated beam oflight for projection over a distance.

Such tubes are of great use in the electronic art, examples beingvelocity modulation tube beampower transmitting and receiving tubes,television projecting tubes, high-intensity X-ray tubes,- and:

other similar devices.

In tubes of the types just mentioned, maximum;

efficiency and control depend, inter alia, upon the production of anelectron beam of relatively small cross section, carrying as high acurrent as pos-- sible at as low a potential as possible, i. e. a rela--tively, low impedance electron beam. In such 14 Claims. (01. 315-16) a 2tron beam. 'Experience with cathode ray tubes incorporating drift tubesof substantial length;

lation tube, show that there should be produced sufiicient positiv ionsto neutralize the charges within the electron beam, provided that thegas pressure be not reduced much below l0 mm;/Hg, a degree of vacuumnear which comtubes the production of a beam of this type is beset withseveral difiicultiesone greatly detri-. mental factor being the tendencyof the beam, once produced, to spread out as it progresses along. thelength of the tube, due to the mutually repellant actioncaused by thespace charge effect of the electrons in the beam.

As one means of preventing this spreading out of the electron beam, theart has depended to a; large extent upon the ionization of molecules ofresidual gas within the tube, the positive ions formed bythe impactofelectrons from the beam.

upon such gas molecules tending to neutralize the negative charges ofthe beam electrons and thus tending to prevent the mutual'repulsionofthe latter. The problem has been complicated by the 4 question of thedegree of vacuum existing within the tube,v thenumber of positive. ionsproduced being dependent upon the number of residual gas moleculespresent. Several factors have contributed to the production of tubeswith increasingly high vacua and atthese higher'vacua fewer positive.ions are produced, so that this remedy for beam-spread has becomedecreasingly potent in its-effect.

.With the advent of tubes embodying afieldfree region, such as the drifttube of a velocity modulation tube itihad been assumed that thegenerally practiced enclosure of such field-free space by'a conductivetube would allow theposi tive'ions produced from residual gas to;collectwithin such tube and therefore wouldcause them to'ex'ert'a greater andmore continued restraining actionupon the undesirable spread of theelecmercial tubes are expected to operate.

The discrepancy between the remedial effect ofgas ionization postulatedby theory and the actual insufficient effect produced in practice hashitherto remained unsatisfactorily explained, although several reasonsfor its occurrence have been suggested. Lackingany satisfactoryexplanation for this undesirable phenomenon, no Way has been' found inthe prior'art of overcoming the same by 30' purely electro-static meansand recourse to electromagnetic concentrating devices has been foundnecessary.

Our invention is designed to overcome, solel by electrostatic means, thebeam-spread difiiculty" 35'- above described and the modus operandithereof is hereinafter set forth.

Ourinvention' has for one object the productlon of an electron beam oflow impedance and the propagation of such beam over a relatively greatdistance through afield-free space enclosed by a conducting shield;

Another object of our invention is the provision of an electricalcircuit and determination of the constants thereof, which'may be appliedto an electron beam tube in order substantiall to reduce or practicallywholly to prevent undesired electron beam spread within such tube.

Yet another object of'this invention is to pro vide a method andmeansfor trapping and confining within a predetermined region positive gasions produced in an electron beam tube.

Yet another purpose of our invention is to provide quantitative datafrom which the design,

construction and operating constants of an eleotron beam tube may bepredicatedin such fashion A still furtherobjectis to produce upon-thetarget or screen of a cathode ray tube, an illuminated spot reduced insize to substantially that of the cross-section of the beam as initiallygenerated, without the necessity of employing electromagnetic beamconcentrating mean's'to secure this result.

Another object is to providemeans and a method for limiting the crosssection of an electron beam to any desired degree commensurate with thecurrent carried thereby, by the employment of purely electrostaticmeans.

Still another object is to provide means for maintaining a denseelectronxbeam'in extremely good vacua, such as 10-? mm./Hg.

Fig. 1 is a diagrammatic representation showing one form of cathoderaytube, embodying our invention and also showing the axial potentialdistribution thereof when connected according to our invention.

Fig. 2 shows a family of curves illustrating the beam-spread effectsobserved-with a tube constructed and operated according to the priorart.

Fig. 3 shows the beam-spread effect due to mutual electron repulsion, inthe absence of positive ion effect, of. an initially parallel electronbeam.

Fig. 4 is a nomograph showing the beamspread effect corresponding .to'agiven potential, total current and beam length in the absence ofpositive ion effect, of an initially parallel electron beam.

Fig. 5 shows graphically beam-spread effects as observed and ascalculated according to the theory of operation of our invention.

In deriving the data upon which our conclusions as to the cause of thedifiiculties encountered in the prior art with electron beamspread werebased and in deriving formulae useful for the design of electron. tubesfor overcoming these difficulties according to our invention, thefollowing conclusions were first reached from theoretical considerationsand were then tested experimentally as to the correctness thereof.

For a typical electron beam, one having the followingcharacteristicswas-chosenas an example:

Area1 cm. Current-0.05 ampere Potential-6000 volts Produced in a aspressure-of mm./Hg

The firstpoint to beconsidered is at what pressure enough gas moleculeswill be found present so as to furnish a sufficient quantity of positiveions completely to neutralize the electron charges of such beam.Formulae well known in the electronic art give the number of electronsexisting molecules to supply a quantity: of-positive ions.

great enough to neutralize all the electron charges present in the beam.

The next question arising is concerning the actual number of ions whichwill be formed under the above stated conditions. This value may befound by the formula set forth by S. H. Bennett in an article entitledMagnetically'self focusing streams and published. in'Phys. .REV., 'vol.45, No. 12, June 15, 1934, pp. 890-897.

."According to this formula the number of ions formedper cm. length ofbeam per second is equalto 200 pi V0 in which .1) is: the pressureexpressed in uum/Hg;

i is the current expressed in E. S. U.; V is the potentialexpressed inE. S. U.; and e i the electroniccharge, also expressed in E. S. U.

Applying this formula to the instant assumed case, it is found that314x10 ions are formed per cm. length persecond. Although the lifeperiod of ions under such conditions and with an allowance for anypossible existence of.recombination, is of the order of one second, theforegoing values derived from the electron count and the speed ofproduction of ions, show that each ion need live only =2.17 X 10 secondsI result in therebeing present too few moleculesto provide the necessaryions for complete electron neutralization. Now commercial tubes ofthe'electron beam types usually operate at pressures ofabout 10- .mm./Hgand yet his well known thatsome tubes actually do display detrimentalbeam-spread effects as soon as the pressure is reduced below 10 .mm./Hg."We have made careful experimental determination upon such tubesprovided with a field-free space or drift tube especially well shieldedand yet :the beam-spread effect reached. noticeable proportionsasisoonas'rth gaspressure was reduced tocarpoint' lying: between. 10-andlo" rum/Hg. i .-In-: Fig. 1 therexisshown as oneexample of ourinvention an electron 'tube having a cathode structure I'll, ;a,.l'fi1$taccelerating electrode ll.

1' held at .1000. volts: positive potential relative. to

\ the seathode, asecond acceleratingelectrode i2,

held 'at. 5000 volts: additional positive potential and. provided withan.apertured end structure l3,:-adrift tube M'helclat about 15 voltsnegative relative to the electrode [2, and-a target or 1001-lectingeiectrode 15, held. at the :same potential aselectrode l2.

The axial potential distribution, :as shown. by thegraph, "startsxatpoint :16 .at zero potential,

rises-asshownv at I! when electrode] I isrea'ched',

rises again sharply as shown at I8, until. at

structure l3'thezmaximum potential. is reached,

showrr'by peak 1:9. There .is .then av reversalof potential gradient .tothe level 20,. existing 2 throughout the vzd'rift tube. l4; and again arise-of gradient to pointizi, corresponding to the .potentialsof target15.8110]; rec ualv to that applied. to..electrodezl 2. v

Fora. betterzunderstandingof the operation of'the device ofeFig. 1,reference is made. toFig.

2, where are plotted the experimentally determined beam-spread effectsfound inan electron tube similar to that of Fig. 1, but having the drifttube maintained atthe same poten-- tial as the electrodes situatedadjacent to each end thereof, respectively, this mode of connectionbeing that commonly used in the prior art.

From the graphs shown in Fig.2 it will be noted as the gas pressure isdecreased while the potential is held constant, the beam-spread effectincreases at a rate which varies according to the particular potentialemployed. The drift tube potential shown in Fig. 1 was varied in thederivation of these graphs. At lower potentials the beam-spread effectis barely noticeable until the pressure is reduced to about 10 mm./Hgand increases comparatively slowly as the pressure is further reduced.At higher potentials the beamspread effect starts at about the samedegree of vacuum, but rises comparatively rapidly to a greater value,reaching its limiting value of approximately R/Ro=3 for a 6100 volt, 100milliamperes beam before the pressure has been reduced by the factor of10. These effects of beam-spread are partially explicable from the factthat over this range of voltages the probability of ionization variesinversely as the voltage and undoubtedly a still higher maximumbeam-spread would be reached if the vacuum were pushed further. When thevoltage is increased above 6100 volts the beam tends to reach itslimiting spread quite rapidly and this limiting spread is considerablysmaller than'that reached around 6100 Volts, this latter effect beingthat which would be expected from ordinary space-charge beamspreadconsiderations, independently of any positive ion phenomena.

As previously stated, mathematical analysis shows that there should besubstantially zero beam-spread at pressures below 10- mm./Hg, due to thepositive ions formed. Yet the results plotted in Fig. 2 clearly indicatethat for some reason the positive ions fail to operate in the mannerassumed by the considerations upon a which the prior art has been based.

In Fig. 3 in which Z is the axial length of the beam there is shown agraph illustrating the theoretical beam-spread effect in the absence ofpositive ionization. Theory shows that spreading beams are reducible tothe shape illustrated by the graph by a suitable change of vertical andhorizontal scale factors. Further in any given structure if thepotential be changed by a given factor the current, if from a spacecharge limited source will in turn change in such a way that the shapeof the beam is not in any way altered. We have experimentally verifiedthis rather astonishing conclusion. The graph of Fig. 3 can likewise beused for initially convergent beams, provided that the beam electronsare initially so directed that in the absence of electrostaticrepulsionthey would converge to a point, since such beams become parallel at somepoint and thereafter diverge.

In Fig. 4 there is shown a diagram which allows the beam-spread efiectto be computed for a given potential, total current and beam length. Thelaws illustrated in Figs. 3 and 4 serve to explain some of thebeam-spread effects shown by the graph of Fig. 2, but still leaveunexplained the occurrence of very appreciable beam-spread at pressureslying between 10- mm./Hg and l-' mm./I-Ig, since considerationspreviously discussed indicate that over this range of pressures thereexists a sufllcient quantity of positive ions confined within-the drifttube, to neutralize com-- pletely the negative ..charges. of theelectrons in the. beam. These considerations, then, which have beenassumed to becorrect by the prior art, show that comparatively tcompleteneutralization ofthespace-charge efiect in the electron beam shouldobviate the occurrence of. appreciable beam-spread at pressures in theneighborhoodof 10- mm./Hg l We are thus confronted with seriousdiscrepancies:v between the assumptions upon which the prior artstructures were designed. and connected for operation,.and the actualresults obtained in practice,- suchresults including the extremelydetrimental and undesirable spreading of the electron beam to a .verygreatdegreawith .consequent failureto .maintain the desired, highdensity and .lowimpedance of the electron beam. After the considerationof several possible tentative explanations for this -discrepancy,'allyielding negative results,twe arrived at the conclusion thatthepositive-ions were removed axially from the portions of the electronbeam adjacent to the entranceend, of the drift tube, and-that as thepositive ions were so removed: other ions. from themorecentral-portionof the beamwere fed axially along the. beam so as to reach the:

portions thereof near ,-the ends of the drift tube, wherethese ions inturn were swept out and removed. This process just described amountsthen to a continual leakage of positive ions along. the electron beam inan. axial direction until such ions reached points lying without thedrift.

tube. This means that complete neutralization of the space-chargeeffects tending to produce,

divergence :of a portion of the electron beam lying within the drifttube, will not take place according to the conclusions above reached,

mathematically; since the quantity of, ions available at pressuresbelow10- -mm./Hg will be insufficient to take care of this. continual axialleakage or loss of ions from. the. interior of the drift tube.

Referring againto Fig. 1, it will be seen that we have established atthe respective ends of the drift tube, a reverse potential gradient. Wehave foundthat this comparatively low reverse potential gradient servesto trap the positive ions within the drift tube and thus substantiallyto prevent the removal of positive ions from the interior of the drifttube by leakage of ions in an,

axial directionalong the electron beam andthus substantially toprevent-beam-spread atthe degree of vacua. employed in ordinarypractice,- for example 10*? rnm./Hg.-v

While we believe-this. theoretical explanation lust given of thetrapping action of our reverse potentialfgradients to be a substantiallycorrect one,. yet-gwe-do not confine. ourselves to such theory, but wehave found by actual successful experimentation, that a cathode ray tubearranged and connected in .--accordance with the principlesillustratedin-Fig. 1, actually does virtually completely prevent theundesirable beamspread effect encountered in the prior art. By

the employmentofourinvention, it is unnecessary to employ *the',magnetic fields found necese sary by the prior art in order to maintaina high density beam having low impedance. 7

According to another viewpoint, an ,explanation of the removal ofpositive ions from the interior ofthe field-:freespaceconstituted by thedrift tube may be found by considering that there exists adjacent thebeam opening atone end of the drift tube an external field whichextends.

.sneaeae oversassma11edistanceiihitoxiitheefleldsireei.spa

: and whichrfield continuously "to :sremov'e -of.;'thesbeam. willbecome-imoreinegative impotential,therebymansihgavalf flovfijdiolthefbfian'hiiflfiiions firomaotherssectinns In other-.xword esmechanismaofmayitbe lconsideredt-in.iitheriguise z'ofrsan ion sinkpointsimtheiel-ectron Omthe'se considrapparen' thatiionireniovalxfromtions it iwi ltabe the electrortibea maytake-iailace overfdistancesfargreater tham heidi-stancetol-whie esac'tnal external: fieldpcnetratesiwithin'theidriftltnbe.

We'fhave de-termined mathematically,- fellowing the'data to beaderivedfromtheaboveitheory, what would :be S-the iappreximate 'vbeamspreadactionof -suchmechanism upon anelectron beam having given-constants. lne-Fig? eareishow'n graphsiillustratingi the beam spreadtaction measure'd=in some oasesand calcnlatedaa'ccording to the mathematicalsnethodsabove referred to; in othencas'e's. -it -will 'be observed thatthemeasuredmnd calculated valuesearedneiose accordance with oneanbthergehusitending to substantiate the I correctness-pf ouritheory ofan ion sink above stated. i

Our mathematieally derived -iconeluslo'ns allow us to predict thegeneral-shape rand "approxim ate values of 'the-curves of==Fie: -'2.risthe voltage-" is lowered-monthly kis'theeffect much th'e'same as anincrease ofpressure wouldihave*had 'inreducingbeam-spread;-but-a1sdthepotexitialgradient -at= thedrift tube apertureis-lowered in direct proportion to' 'the voltage=-decrease;thus stillfurther acceri tmating the efie'ct. The -effect will be at first almosta function off-voltage squared; thusagreeing with the measured resultsshown in Fig. 2. ==At*the'higheriran'gescfvoltage,

the maximum-- spread inia perfect Va'cuum would become less and '"ss asthe voltage is ra-ised. These two effects ogether explain why highervoltage-beams-reach. heirulti'mate spreads more quitalelyastheg therespective timate walues of --'spread; grow progressively less,- a as-the voltage rises.

In carryingioutiiexperinrenta tests serving to substantiate thecorrectness o'f our:theory-'-and serving as a basis for the embodimentof'wo'ur invention, apparatusarranged-as"shownin'Fig; 1 was used."Thevnpward lrihks*1 and 21 of the axialpoten-tialcurve=were*obtainediby-maintainingthe-drift tube at --'a;potential*negative "with respect to boththe secondaccelerating-"electrode and' 'the' collecting electrode;by'an'amountslightly greater than the voltage existing between the edgeand the-center-'of= the*beam'wherr the latter consists only ofunneutralizedlectrons. Due to the fact that. 'the' drift--tubecollects acurrent of stray electrons, merely-placing aj-re'sistorin an externalcircuit 'between "the drift tube and the :aperture --providesthe-necessary *potential. Should stray 'eleetroncurrerit--prove-=insufiicient for this purpose, an auxiliaryvoltagesupply could be used.

immediately stoppedspreadingWhea es little as ressureisadecreased;'butthat 1'5svolts was applied between drift tubezand adjacents-electrodes in .such direction as to reverse the gradients atthesepoints. Since 15 volts variationgfrom 7300 can haveno .appreciableelectronilens 0r focusing efiect per se, this result may be consideredas successfully verifying thetheory. Values of the spread variation withchange of trapping voltageare given in Table I, below.

"Table'I.--Beam spread ion trap data Asjan auxiliary check, increasingthe pressure to 5X10 also served to remove thesprea'd as illustrated inTable II, below.

Table IIBeamspread data Of course, in actual use increasingthe pressureis impractical since tubes are normally used as sealed off devices, sothis latter Table II merely servesato demonstrate that the efiect ofextremely good vacuum inproducing beam spreads could be removed by theuse of anion trap .as-suggested here.

It: was noted in the test of ion trap voltage just described. that forthe particular beam used 15 volts removed practically all spreading andhigher negative voltages produced very little increased efiect, but nodetrimental effects were observed when such higher negative voltageswere used. It is to be remarked that this voltage is only slightlygreater thanthe difference in voltage-between the edge andthe center ofthe beam. when it consists of unneutralized electrons only. This voltagedifierence'for .any section s on thebeam would be where p is thenumber'of unneutralized electrons .a, negative gradient down which ionscanifiow will-always be present until thedrift-space is loweredby thisten volts plus enough more to reverse the gradient produced by fieldpenetration.

-As shown by the-above 'Table I,-the voltage difference betweenthe-drift tube and its ends (back plateand aperture) to form an ion'trapmustbesomewhatgreaterthan 10 volts for e milliampere- '6000-volt beam. 1The value necessary-would increase directly as the current 'inthe-beam'and vary inverselyas'thesquare root ,9 of the voltage of thebeam. Thus for volt, 500 ma. beam it would be $3 33 144 volts For a 5ampere, 6000 volt beam it would be 5o 1 V For beams of this magnitudethe ratio of radii of the beam and surrounding drift tube also bex 1000volts comes of some importance.

necessary is given by the following relations for a 6000.volt beam: Letthe volume available=N cm. =Sem.Acm. .1 Let the length of the beam =Scm.1.

Then

i amperes (Where the volume used has the same length as the beam, Agives the area necessary.)

Thus a 1 ampere beam at 10- mm./Hg needs only a volume the length of thebeam with an area of 3.75 cm. For voltages other than 6000 volts thisvolume should be multiplied by What we claim is:

1. An electron beam tube including means for generating electrons, meansfor producing a beam of said electrons, means for accelerating saidbeam, means for defining a substantially field-free space through whichsaid beam passes,

'means for collecting said beam after, passage through said field-freespace, and means for maintaining said field-free space defining means atan electrostatic potential slightly negative with respect to saidcollecting means and with respect to the portion of said acceleratingmeans adjacent said defining means.

2. The method of substantially preventing beam-spread of an electronstream within a field-free space which includes directing a beam ofelectrons within said space, establishing and maintaining a reversepotential gradient at least at one end of said field-free space, so asto trap positive ions therewithin.

3. A cathode ray tube system including means for generating and forcollecting an electron stream in an atmosphere of not more than 10"-mm./Hg, a field-free drift tube through which said stream passes, andmeans for maintaining the direct current potential of said drift tubetive with respect to said accelerating electrode so as to preventleakage of positive ion from said 10 drift tube. and means forcollecting the electrons.

5. In the electron beam art, the method of pro- .iecting a beam over adistance without substantial spread thereof which includes the steps ofestablishing a substantially'field-free space, passing through the spacesaid beam, and establishing at each end said field-free space of arelatively slight potential gradient in the reverse direction to thenormal potential gradient along the course of said beam, wherebypositive gas ions are held within said field-free space so assubstantially to neutralize the space-charge efiects tending to causespread o'f'said beam.

'6. A vacuum tube system including an accelerating electrode, afield-free drift tube insulated from other electrodes and means forkeeping said drift tube slightly negative with respect to saidaccelerating electrode so as to prevent leakage of positive ions fromsaid drift tube.

7. In electron optics, a system for reducing the spread of an electronstream due to space charge effects, including a source of positive ions,a fieldfree chamber within which the ions are produced and through whichsaid stream passes, and means for retaining said ions within saidchamber, said last means comprising electrostatic means so connected asto establish at least at oneend of said chamber a slight di'rectcurrentpotential gradient along said stream in a direction reverse to thegradient prevailing along said stream before the entrance thereof intosaid chamber.

8. The method of passing an electron beam through a field-free spacewithout substantial beam-spread, which includes passing said beamthrough a slight reverse direct current potential gradient, immediatelypassing said beam through said field-free space and supplying gasmolecules to said beam during its passage therethrough, whereby positiveions are formed, are substantially confined within said field-freespace, and act substantially to neutralize the space-charge effecttending to cause spread of said beam.

9. A system for projecting a low impedance electron beam includingconductive means defining a field-free chamber through which said beamis projected, gaseous means within said chamber producing positive ionsunder the impactof said beam, and electrostatic means maintaining at oneend of said chamber a direct current potential gradient along saidstream in a reverse direction to the potential gradient used inestablishing said stream, whereby said ions are restrained within saidchamber and act to neutralize the spacecharge effect tending to spreadsaid beam and increase the impedance thereof.

10. An electron beam tube for the production and projection of a beam ofhigh density, including an electron source, an accelerating electrodeestablishing the beam, a drift tube through which said beam passes,means for keeping said drift tube at a slightly negative fixed directcurrent potential with respect to said accelerating electrode and meansfor collecting the electrons of said beam after passage thereof throughsaid drift tube, whereby said negatively charged drift tube tends toretain therewithin positive ions formed from the residual gas within thebeam tube.

11. A cathode ray tube system including means for generating an electronbeam, means for accelerating said beam, means for projecting said beamthrough a tubular electrode and means for main taining said tubularelectrode at a direct current potential negative with respect to thepotential of said accelerating means adjacent said tubular electrode byan amount which is less than one 121 half. of one percent ofthepotentialwof said lastmentioned: accelerating; -means- 'so. -astoprevent leakage ofv positive ions:- from :said 1 tubnlan electtrode.

12. A cathode ray tubessystem includingrmeans for generating, an.electron beam, .means==for accelerating said beam, meansforzprojectingasaid beam through atubular electrode; means-forcoilecting electrons from said :heam -after.its passa'ge through saidtubular: electrode, and means for maintaining saidtubularelectrodeiat-a'direct current potential negativeewithrespect tosaid col lecting means by an amountwhich islessthan one-half of onepercentof the. :potential of said.

collecting means so as to prevent leakage of positive ions from saidtubular electrode.

13. A cathode ray tubesystemdncludingmeans for generatingan-electron-Joeam; means for acceltrating said beam, meansfor'projecting, said beam through a tubular electrodeand-means formaintaining said tubular electrodeaha-slightly nega tive direct currentpotentialwith respect to said acceleratingmeans, the potentialof said.tubular electrode being sufficiently'negative withwespect to thepotential to said-accelerating means: to prevent leakage ofionsfrom-within said tubulartelec trode vbut not sufficiently-negativeto substantially retard the speed-0f the electrons ,passingtthrough saidtubular electrode 12 14. A mztlmde ray/tube system according to claim 13wherein the voltage diflerence between the voltage of the said tubularelectrode and said acceleratingjmeans is .p 6:94X10" where p is thenumber of unneutralized electrons per centimeter length in the beam atthe section where itkpassesthrough ithe tubular electrode.

SPANGENBERG.

LESTER M; FIELD. ROBERT HELM.

REFERENCES 7 CITED Theiollowing references areof record in the flle o'fthis patent:

I UNITED STATES PATENTS 7 Date varianjhhnncu..- June 17, 1941

